Secure tags are used for a number of different purposes; a primary purpose being preventing, detecting, and/or deterring counterfeiting of an item to which the secure tag is affixed.
One type of secure tag that has recently been developed includes multiple small particles of a host (such as glass) doped with one or more rare earth ions (“RE ions”). This type of secure tag is described in US patent application number 2004/0262547, entitled “Security Labelling,” and US patent application number 2005/0143249, entitled “Security Labels which are Difficult to Counterfeit”, both of which are incorporated herein by reference.
These RE particles can be applied to valuable items in different ways. For example, the secure tags can be incorporated in fluids which are applied to valuable items (by printing, spraying, painting, or such like), or incorporated directly into a substrate (paper, metal, rag, plastic, or such like) of the valuable items.
In response to suitable excitation, a secure tag comprising RE particles produces a luminescence spectrum having narrow peaks because of the atomic (rather than molecular) transitions involved. The narrow luminescence peaks result primarily from internal (4f to 4f) transitions of the lanthanide ion. Luminescence is a generic term that relates to a substance emitting optical radiation in response to excitation, and includes photoluminescence.
Photoluminescence is a generic term that includes fluorescence and phosphorescence, which will now be described with reference to FIG. 1, which is a simplified Jablonski energy diagram 10 showing most of the possible transitions in a molecule or atom. In FIG. 1, the wavy lines represent dark transitions (transitions that do not emit or absorb light). The solid lines represent transitions that absorb or emit light.
The molecule or atom starts out in the ground state (S0) 12. When the atom or molecule absorbs light of the appropriate frequency (illustrated by arrows 14 in FIG. 1), electrons in the molecule or atom are promoted to a first singlet excited state (S1) 16 or to a second singlet excited state (S2) 18 (each state having multiple vibrational energy levels). The spin on the promoted electrons are preserved during excitation. The electrons are typically excited to a higher vibrational energy level in the first singlet excited state (S1) 16 before rapidly relaxing (illustrated by arrows 20 in FIG. 1), to the lowest energy level in the first singlet excited state (S1) 16. This event is termed vibrational relaxation or internal conversion and occurs in about a picosecond or less. The excited state may decay directly back to the ground state by way of fluorescence (illustrated by arrows 22), quenching (illustrated by arrow 24), or non-radiative relaxation (illustrated by arrows 26). The excited state may also transfer energy to the triplet excited state (T1) 28, which is referred to as intersystem crossing, as illustrated by wavy line 30. The spin on the electron is flipped as it moves from S1 to T1. From the T1 state the molecule or atom may emit a photon of light (phosphorescence) 32 or lose the energy via non-radiative relaxation 26. During phosphorescence the spin on the electron is again flipped. The transition from T1 to S0 is slow compared to other possible transitions, the timescales are typically between 10−3 to 102 seconds. Thus, in internal conversion the spin is preserved; whereas in intersystem crossing the spin is flipped.
Secure tags based on RE ions phosphoresce, which allows a delay to be used between excitation and measuring the stimulated phosphorescence. This ensures that any fluorescence from background material (such as a substrate on which the secure tag is located) has decayed prior to the phosphorescence measurements taking place.
To enable quick and accurate validation of a secure tag, a luminescence signature is typically derived from the luminescence measured from that secure tag. This luminescence signature may be based on peak locations, absence of peaks, relative peak intensities, and such like. A luminescence signature is typically derived by converting a large number of data points from a luminescence spectrum into a relatively short code. This short code (the luminescence signature) enables rapid comparison with other, pre-stored luminescence signatures to facilitate validation of the secure tag.
It would be desirable to increase the security of secure tags based on RE particles to make them even more difficult to counterfeit, without making validation of the RE particles slower or more expensive.